1. Field of the invention
The present invention relates to an LED (light emitting diode) display having LED elements in a matrix arrangement separated by a plurality of stripes of grooves.
2. Description of the Prior Art
Since in a direct-view display and a projection matrix display, LED elements are arranged close to each other, an LED display of a monolithic type as disclosed in Japanese Patent Laying-Open Gazettes No. 56796/1979 and No. 136788/1985 is preferable to that of a hybrid type.
Referring now to FIGS. 1A and 1B, an LED wafer having a p-n junction 210 is mounted on an insulating substrate 100 of ceramic or the like, a plurality of stripes of grooves 240 and 250 which are deeper than the p-n junction 210 are provided orthogonal to each other from the surface and then, LED elements are connected by thin metal wires 300 in the row (or column) direction. At that time, each of the LED elements separated by the grooves 240 and 250 serves as picture elements 261, 262 and 263.
However, in such a method, the LED elements 261, 262 and 263 are often damaged, because a compound semiconductor which is a material for the LED wafer is brittle. More specifically, undesired cracks are caused and the LED elements are stripped off due to vibration or tortion of a cutting tool such as a dicing blade during formation of the grooves and shocks during wire bonding, as seen in FIG. 1A. The LED elements 261 located in the outermost peripheral rows or columns are damaged more often than the LED elements 263 located in the center. The grooves 240 and 250 are provided after a wafer is mounted on the substrate 100. Since the area occupied by the mounted wafer is large, electrically conductive adhesive 500 is squeezed out and climbs along the side surface of the semiconductor crystal as seen in FIG. 1B. As a result, the p-n junction 210 is liable to be short-circuited.
If the above described defects are caused, an image indicated on the LED display will be imperfect. However, the LED display can not be repaired even if the LED elements located in the outermost peripheral rows or columns are damaged, because each of the LED elements is not of an independent pellet. Therefore, even if one of the LED elements is damaged, the whole matrix is considered as an inferior product, so that the yield thereof is less than 5%.
In such an LED wafer of a matrix type, light from a lighted LED element is reflected by LED elements around the lighted LED element, as shown in FIG. 2. The lighted LED element looks large, while the LED elements located in the outermost peripheral rows or columns look small because the outermost side walls are opened, so that the image quality on the LED display is not high.
When the grooves 250 are formed, a cutting tool such as a dicing blade touches the substrate 100. Therefore, if the substrate 100 is made of a hard material such as ceramic, the LED wafer may be damaged by chippings. In addition, the lifetime of the cutting tool becomes shorter and the blade is liable to be nicked. On the other hand, if the substrate 100 is made of a resin plate or a surface-treated metal plate, the grooves 250 are easily formed. However, the LED generally generates heat as it is lighted. Since in the LED display of a matrix type, the LED elements are arranged close to each other, the heat is concentrated on the substrate 100. Therefore, when the substrate is made of a resin plate, the output of light is decreased and the LED elements may be stripped off because a heat radiation characteristic of the resin plate is bad. On the other hand, when the substrate is made of a metal plate, the LED elements may be stripped off due to the difference in coefficient of thermal expansion between the metal plate and the LED matrix.